The present invention is related to the art of growing silicon films on ceramic substrates by bringing the substrate in contact with molten silicon. The present invention is broadly related to prior art U.S. Pat. Nos. 4,112,135, 4,128,680 and 4,137,355 by Joseph D. Heaps and Obert N. Tufte, all assigned to the same assignee as the present invention, where there is described apparatus and method of coating ceramic bodies or sheets with molten silicon to prepare large area, thin sheets of large grain polycrystalline silicon on inexpensive ceramic substrate for use in solar cell panels and the like. This is referred to as silicon-on-ceramic or supported growth techniques. In that method the side of the ceramic sheet or the area to be coated with silicon is first carbon-coated. It is taught in the cited references that when a ceramic (which normally is not wet by molten silicon) is first carbon-coated on a surface to be silicon coated, the carbon-coated surface will then be wet by molten silicon, and by contacting the carbon-coated ceramic substrate with molten silicon, a silicon coating will be formed thereon. The present invention is also generally related to copending application Ser. No. 79,844, filed Sept. 28, 1979 and assigned to the same assignee as the present invention.
The present invention is also directed to growth of sheet silicon on a substrate, specifically to a method called "cold substrate growth". By the term cold substrate growth is meant growth of sheet semiconductor (e.g. silicon) under conditions such that the latent heat of freezing is conducted directly to the substrate, through the previously solidified solid.
The improved growth technique lies in what may be described or classified as an asymmetric mode of growth of the silicon-on-ceramic in contrast with the symmetrical mode of growth occurring in the process of the prior art patents cited above. This asymmetric mode of growth of silicon on the substrate is so classified because the substrate must remain cooler (i.e. at a temperature cooler than the solidification temperature of silicon) in the area of the substrate where the silicon layer growth is taking place so that the liquid-solid interface is nearly parallel to the substrate, but inclined as an angle, so the leading edge of the crystallization front (the first liquid to solidify) solidifies on the substrate.